/Engine/Source/Runtime/Slate/Public/Widgets/Layout/SSplitter.h
Given the index of the dragged handle and the children, find a child below/right_of the dragged handle that can be resizedChildren.Num()if no such child can be found.
static int32 FindResizeableSlotAfterHandle ( int32 DraggedHandle, const [TPanelChildren](API\Runtime\SlateCore\Layout\TPanelChildren)< [FSlot](API\Runtime\Slate\Widgets\Layout\SSplitter\FSlot) > & Children )